Preamp Cathode Resistor values, AB763 - 12AX7

[orrong65]

I have added the Diaz Vibroverb cathode mod to a number of blackface amps with great success.

I add a 2-way toggle switch to the back panel, one setting is stock, the other is Diaz mod just like the CS Vibroverb RI. I only add the cathode switching, and not the channel and tremelo disconnect.

The mod is 22uf/2.7k on V2a (22uf/1.5k stock) and 0.68uf/2.7k on V2b (22uf/1.5k stock) on the vibrato channel, which is voiced brighter than the normal channel.

This mod delivers more marshall-like treble tone and much more distortion at overdriven levels, perfect for SRV tones.

[guitarmike2107]

When you bias colder you will get a slightly more compressed tone, its tighter and is generally better for giving a tighter driven sound.
This is another reason why most high gain amps use colder bias.. Compression + Gain =
Visually this compression can be seen by the way the grid curves are bunched up on the lower right hand side of the plate characteristics charts.

But on clean amps I think to many cold stages lacks warms, so biasing the first stage hotter can help warm up a clean channel a little.
But as you warm it up it may also start to loose the punch that the compression gives it.

You have to try it, the results can be very subtle but they are there

Biasing hotter or colder from the centre biased position will increase the triodes sensitivity, i.e. the valve will distort easier with the same input voltage.